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The retina as a window to the brain from eye research to CNS disorders
Anat London, Inbal Benhar and Michal Schwartz
Abstract | Philosophers defined the eye as a window to the soul long before scientists addressed this clich to determine its scientific basis and clinical relevance. Anatomically and developmentally, the retina is known as an extension of the CNS; it consists of retinal ganglion cells, the axons of which form the optic nerve, whose fibres are, in effect, CNS axons. The eye has unique physical structures and a local array of surface molecules and cytokines, and is host to specialized immune responses similar to those in the brain and spinal cord. Several well-defined neurodegenerative conditions that affect the brain and spinal cord have manifestationsin the eye, and ocular symptoms often precede conventional diagnosis of such CNS disorders. Furthermore, various eye-specific pathologies share characteristics of other CNS pathologies. In this Review, we summarize data that support examination of the eye as a noninvasive approach to the diagnosis of select CNS diseases, and the use of the eye as a valuable model to study the CNS. Translation of eye research to CNS disease, and deciphering the role of immune cells in these two systems, could improve our understanding and, potentially, the treatment of neurodegenerative disorders.
London, A. etal. Nat. Rev. Neurol. 9, 4453 (2013); published online 20 November 2012; doi:10.1038/nrneurol.2012.227
Introduction
Cicero (10643BC) was first to declare ut imago est animi voltus sic indices oculithe face is a picture of the mind as the eyes are its interpreter. Many different versions of this phrase have since been quoted by philosophers, orators, politicians and writers. Over the years, scientists have struggled with the concept that the eyes are the window to our soul, searching for scientific evidence to determine whether eye research could be useful for investigating the brain and diagnosis of its diseases.
During embryonic development, the retina and optic nerve extend from the diencephalon, and are thus considered part of the CNS. The retina is composed of layers of specialized neurons that are interconnected through synapses. Light that enters the eye is captured by photoreceptor cells in the outermost layer of the retina, which initiates a cascade of neuronal signals that eventually reach...